This invention pertains to data storage devices using as the recording medium disks coated with thin layers of magnetic material in which magnetic patterns may be created by a data transducer in relative motion with respect thereto. In the high data capacity implementation in which the invention may be used, the disks are made of a rigid material, usually aluminum, and are mounted for high speed rotation on a spindle. Usually in such a device a number of disks are carried on a single spindle. The spindle itself is mounted for rotation on a deck which carries the various subsystems of the complete drive unit. A motor either within the spindle or mounted below the deck provides torque for rotating the spindle.
The transducers which record the data on the disk and read it back are carried on arms which suspend them adjacent the disk surfaces. The arms are in a currently preferred design fixed to a central frame or body, all of which together form an arm assembly which is mounted for rotation on the actuator shaft adjacent the edges of the disks. The actuator shaft is fixed to the deck. The arm assembly is rotated by an adjacent actuator to position the heads at any desired radius with respect to the disk spindle axis. As the disks rotate, the transducers trace circular tracks on the disks concentric with the disk spindle axis, and it is in these tracks that data is recorded. Thus, for each arm assembly angular position, access to a number of tracks and all the data recorded in them is possible.
It turns out that design improvements which increase the amount of data recordable on a single disk surface of a particular size are cheap relative to the value added by the increased data capacity. One innovation which has substantially increased the density of data on disk surfaces involves the dedication of one disk surface to so-called servo data which is permanently recorded with great accuracy thereon in concentric servo tracks. The transducer assigned to the servo surface functions only for reading the servo data, and provides servo signals for a feedback control loop whose output is to the actuator which angularly positions the arm assembly. By properly interpreting these servo signals, the servo circuit can cause the actuator to either shift the servo head into a position above any desired servo track or, after doing this, enter the so-called track following mode which maintains the servo head in very accurate registration with the desired servo track. Even though there is unavoidable radial runout in the disk spindle of perhaps hundreds of .mu.inches, the servo feedback control loop can maintain the servo transducer such that deviation from perfect registration of the servo transducer with the desired servo track is at worst one or two tens of .mu.inches.
The servo loop and other technology relating to these disk-type data storage devices has now reached such a level of refinement that even small devices, say those having the popular 5.25 inch diameter disks, can store upwards of 10 megabytes (Mb) of data per disk surface. To achieve the stated quantity of data per disk surface, the data must be packed with very high real density. Thus, the number of separate tracks per radial inch must be very high, and in the present designs, is 1000 tracks per radial inch or higher.
It is important that the data transducers all be constantly maintained in precise alignment or registration with the desired data tracks, because if the alignment is not accurate, data cannot be read and recorded accurately. If misalignment between a data transducer and the desired track exceeds perhaps 15% of the track spacing, errors are likely to occur with unacceptable frequency. Thus, for a track density of 1000 tracks per inch (tpi), the maximum allowable misalignment between the individual data transducer and the nominal position of a data track to be read and recorded is 150.mu. inches (approximately 4 .mu.m.). Since an error in writing a particular data track may coincide with a similar error of opposite sign during reading, in fact the actual error must be held to approximately .+-.75 .mu.inches, allocating the permitted error equally between the write and read operations.
The accuracy with which the servo transducer is held in alignment with a servo track by the feedback control loop makes this particular source of error a relatively minor cause of misalignment between the data transducers and their data tracks. There are, however, a number of other sources of alignment errors of the data transducers with the data tracks, among them being thermal change in disk radius, thermal tilt of the disk assembly, vibration of the transducer supports, and bearing runout. Many of these errors have been dealt with satisfactorily one way or another or have been found to be relatively minor.
In one particular disk drive design with which the inventor has worked, the single most significant source of data transducer misalignment with the data tracks arose from thermally induced misalignment of the servo head with the data heads. For the arm assembly design used in this disk drive unit, it has been determined that changes in the temperature within the enclosure housing the disks and arm assembly causes angular change in the alignment of the arms with respect to each other leading, of course, to misalignment of the heads with respect to each other. Such direct thermal effects are not uniform from individual arm assembly to assembly or from individual arm to individual arm in the same assembly, for some assemblies being as little as 80 .mu.inches and in others as much as 180 .mu.inches. In particular, it was found that the arm closest to the deck and the transducer carried on it was by far the most affected, with the alignment with respect to each other of the other arms and the transducers carried on them being relatively unaffected. The fact that the arm closest to the deck was most affected had a disproportionate effect on the misalignment of the data transducers with their tracks during track following operation because for reasons not important here, the transducer closest to the deck must be used as the servo transducer.
The reasons for such temperature-induced misalignment is not clear. It is known that the arm assembly is not bilaterally symmetrical, and this undoubtedly has a substantial effect. But theoretical calculations do not derive errors of this magnitude and in any case, do not explain the substantial variation from one assembly to another. It is clear that this effect is amplified by some other mechanism. One likely source is the high axial compressive loads placed on the arm assembly bearings during assembly. These preset loads are necessary to reduce radial runout in these bearings, which if present can cause very large positioning errors. Since it is difficult to tightly control the amount of axial load during manufacture, there is a wide range in load in installed arm assemblies. The arm assembly temperature changes causes changes in the length of the arm assembly. The temperature-induced arm assembly length changes can substantially change the loads on the arm assembly. These changing arm assembly bearing loads in conjunction with the asymmetry of the arm assembly could be the source of these temperature-dependent transducer misalignments. The difficulty of consistently establishing the initial loading could account for the differing amounts of maximum misalignment from arm assembly to arm assembly.
Whatever the mechanism, it is clear from statistical studies that the amount of misalignment between the servo arm and the data arms changes in approximate direct proportion to the change in temperature in the enclosure regardless of the maximum misalignment observable for a particular arm assembly. Further, it has been determined that the alignment error is always of the same sign regardless of the arm assembly unit involved, i.e. a data head can be observed to shift out of alignment with its data track in the same direction for every arm assembly. Thus if a data track is written at a particular temperature, immediately thereafter no temperature-induced misalignment between the data arm and servo arm will be present to cause misalignment between the data transducer and its track. If the misalignment between this particular data arm and the servo arm is then measured after the temperature has risen, say 10.degree. F. a certain amount will be detectable. Its magnitude cannot be predicted a priori with total accuracy but its sign or direction will be known to be that which has been experienced with every other arm assembly of that design. Further, it can be reliably predicted that the misalignment is approximately linear with temperature changes. That is, for a 20.degree. F. temperature rise for example, the misalignment will be very close to twice that for 10 .degree. F., and for a 5.degree. F. temperature rise, approximately half that for 10.degree. F. For temperature decreases, the misalignment will be in the opposite direction but still will have this proportional response to temperature changes. The amount of change in misalignment for a given temperature change will vary from arm assembly to arm assembly, but the change will nonetheless be proportional with respect to temperature.
Temperature changes in the enclosure occur of a variety of reasons. The ambient temperature may change. The enclosure slowly heats up as the unit remains in service for a length of time due to heat from the electronics and resistance losses in the actuator. Mechanical energy present in air turbulence and bearing friction is also converted into heat.
It should be noted that the initial alignment of the servo transducer with respect to the data transducers is irrelevant to transducer to track alignment during operation. The alignment of data transducers to data tracks is set when data tracks are first recorded, and only changes in alignment between the servo transducer and the data transducer after it records a track can cause transducer misalignment with a data track. Of course, after a first data track is recorded on a disk surface, it is necessary that the adjoining tracks be properly positioned with respect to it to prevent their interfering with each other.
The closest art known to the inventor are the following U.S. Pat. Nos.: 4,185,309; 4,194,226; 4,135,217; 3,720,930; 3,871,064; 3,872,575; 3,775,655; and 3,029,318. Xerox Disclosure Journal, Vol. 5, No. 5, Sept./Oct. 1980, p. 549, Thermal Servo System for Mechanical Positioning, is also of interest.